Plane on a conveyor belt
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I think I finally see where we are butting heads, Tjamz; in your example you assume that the wheels are frictionless; if in fact the wheels were frictionless, then yes the plane would be able to move.
I interpreted the original scenario to be not an ideal world but one in which friction exists. The plane is set such that it's velocity relative to the belt surface is perfectly counteracted by the velocity of the surface. Thus, the plane is stationary with respect to the air, with its thrusters on (after all, isnt that what is moving the plane in the first place?). If this wasn't the case and the plane would be stationary WITH or WITHOUT its thrusters then this really isn't a Physics problem at all, but common sense. A guy on an ice rink holding a big fan would start sliding. It would make more sense to have the question set to where the speed of the plane is matched in an opposite fashion (that is, the belt is actually retarding the advance of the plane).
If the situation was how I interpreted earlier then you would have two outcomes:
Case #1: Wheels are not frictionless Answer = no, the plane doesn't move (as I have been trying to get at). There are static frictional forces that the plane must overcome, and if it is already giving 100% thrust and not going anywhere, it isn't taking off). The postulate here is that the treadmill is imparting enough drag on the plane via friction to counteract the thrust.
Case #2: Wheels are frictionless (belt can do no work) Answer = yes, the plane can take off. If world was frictionless then the surface would not matter. The thrusters might as well be off, for the belt moving wouldn't affect the plane at all.
This question is hard due to its ambiguity, not its physics setup.
2005 Mercedes-Benz C240 4Matic
1993 Mazda Rx-7 Twin Turbo (sold)
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I posted this on another forum; and it seems it's just a never-ending argument! Where is the "PROOF" that it'll FLY or NOT? so it can end the madness
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AcesHigh wrote:
I think I finally see where we are butting heads, Tjamz; in your example you assume that the wheels are frictionless; if in fact the wheels were frictionless, then yes the plane would be able to move.I interpreted the original scenario to be not an ideal world but one in which friction exists. The plane is set such that it's velocity relative to the belt surface is perfectly counteracted by the velocity of the surface. Thus, the plane is stationary with respect to the air, with its thrusters on (after all, isnt that what is moving the plane in the first place?). If this wasn't the case and the plane would be stationary WITH or WITHOUT its thrusters then this really isn't a Physics problem at all, but common sense. A guy on an ice rink holding a big fan would start sliding. It would make more sense to have the question set to where the speed of the plane is matched in an opposite fashion (that is, the belt is actually retarding the advance of the plane).
If the situation was how I interpreted earlier then you would have two outcomes:
Case #1: Wheels are not frictionless Answer = no, the plane doesn't move (as I have been trying to get at). There are static frictional forces that the plane must overcome, and if it is already giving 100% thrust and not going anywhere, it isn't taking off). The postulate here is that the treadmill is imparting enough drag on the plane via friction to counteract the thrust.
Case #2: Wheels are frictionless (belt can do no work) Answer = yes, the plane can take off. If world was frictionless then the surface would not matter. The thrusters might as well be off, for the belt moving wouldn't affect the plane at all.
This question is hard due to its ambiguity, not its physics setup.
You ALMOST got it. How about case #3 where the bearing in the wheel and the wheel itself do provide friction, but not so much friction that thrust from the engines cannot overcome it and achieve the forward momentum required for the air to flow over the wings and cause lift.
In your case #1 example, it is impossible for the plane to take off under any circumstances as you are assuming that any friction put on the wheels/bearings is enough to keep it from moving forward under thrust from the engines when in reality the most resistance you are encountering is just to get the plane moving, once it is moving the resistance becomes almost a complete non factor in the equation as there isn't a significant amount of increase in resistance to spin the wheels at 100 mph than there is to spin them 200mph (the conveyor belt is the only thing roll the wheels directly btw...in a normal situation the relationship of the engines (jets, props, whatever...when I say "engine" those are what I'm refering to) pushing the plane forward on the ground would be causing them to roll.
I'm still right.:D
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camzaro28 wrote:
haha, yeah once i read dave's post i understood. i was under the assumption that no matter what the plane was at a standstill and was gonna take off like that! i read over it a lil to quick. ah, oh well. i get it now.YES the plane will take off. maybe i should have read the 9 or so pages b4 posting
I'm really glad you changed your mind.. CMK decided to call me this afternoon to inform me that you believed the plane would not take off....and Brian, I lost a lot of respect for you for a while there.
I'm done replying phisics bullshit....
I'll lose my mind over it.. -
heh, I'm arguing this point on like 6 forums now....two for airliners, here, MNSC, MisterCMK, and a physics board....fun stuff.
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And what are the outputs from the airliner forums? j/w!!!
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people on both sides of the fence there as well, kinda sad to think that a lot of our pilots seem to think that if their wheels are being spun they can't generate any forward thrust. The key to remember here is that the wheels spin in the same direction whether they are being driven by the thrust of the jet/prop or the conveyor belt...or both. If both forces are acting upon the wheels they double in the amount of rotations they'll make in a given period of time.
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http://www.avweb.com/news/columns/191034-1.html
This guy knows what he is talking about.
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chiming in late here but i think people are looking to deep into it, air speed and ground speed are 2 different things, the landing gear wheels should be thrown out, because they are nuetral doing what ever forces acting apon them want them to causing no affect on the speed of the plane or the conveyor
say the plane needs 50mph of air flying by it to create lift, being the wheels are nuetral why would 50 mph of payment moving under it in the other direction causing the plane to act any differently
think of it this way remembering that air speed and land speed are totaly seperate things
super man is flying along at 50mph of AIR SPEED with a ball bearing pizza cuter in his hand, now flying at this speed he finds a 50 foot long tread mill moving at 50mph LAND SPEED in the other direction
as he fly's by this this tread mill he reaches out and touches the pizza cuter to the moving belt, now does he just stop in mid air?
NO, other than the friction of the pizza cuter his air speed will stay un-affected
but the real question here is how long does it take super man to flay by this 50 miles long tread mill??
makes sense to me but having a simple mind some thing just come more simple to me
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I feel sorry for you guys that don't get this when Schell does.....j/k bud, good analogy
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Damn, I missed out on this argument. Thanks to all of my advanced mechanical engineering classes...Wait, I don't need any of them. It flys. It would be the same as with a runway that is stationary. The only small difference is the greater rotational friction force from the wheels spinning faster, which is going to be relatively small. Planes move from thrust not like a car...
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tjamz wrote:
In your case #1 example, it is impossible for the plane to take off under any circumstances as you are assuming that any friction put on the wheels/bearings is enough to keep it from moving forward under thrust from the engines when in reality the most resistance you are encountering is just to get the plane moving, once it is moving the resistance becomes almost a complete non factor in the equationFuck. You got me there. I was imagining a scenario where static friction is strong enough to hold the plane in place. I reread the first post and saw that the belt moving at the same speed as the plane, in which case it wouldn't. Bastard..
2005 Mercedes-Benz C240 4Matic
1993 Mazda Rx-7 Twin Turbo (sold) -
YAY, I WIN HERE TOO!!!
I think I just ended a 130+ page long argument on phys.org with my analogies as well. I rule!
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well, Mythbusters is going to take on this one on January 30th....just thought I'd bring this thread back from the dead for some good arguments til then.
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AFAIK it depends. I am assuming treadmill is movig at the same speed as the engines would be propelling the aircraft forward, thus making the aircraft basically stationary on the treadmill with the wheels spinning. If the airplane has strong enough engines to pull the entire weight into the air without the aid of the lift created by the wings then it will fly, but the engines would have to be pointed at an angle to go 'up'. in this scenerio the aircraft would also be able to lift itself into the air from a standing stop without a treadmill, like a VTOL. in the case of a normal passenger aircraft, where the engines themselves are not enough to pull the aircraft into the air, the aircraft needs to be moving forward at a certain rate of speed (say 150mph) for the wings to create enough lift to get it off the ground. In that case, if the treadmill is keeping the aircraft from moving forward and creating lift, the aircraft will not take off. if you did the same thing in a wind tunnel however, with enough wind to create enough lift under the wings, the aircraft would fly. also if the treadmill were as long or longer than a runway and the engines of the aircraft were somehow strong enough to pull the aircraft forward at takeoff speed against the treadmill (so on a 50mph treadmill, 200mph assuming a 150mph takeoff speed) but not enough to lift the aircraft without the aid of lift created by the wings, it would fly once it reached takeoff speed. just the way i think about it.
as i understand it, most aircraft stay in the air via lift via thrust. the engines are not strong enough to pull a gigantic jetliner into the air from a standing stop. instead, the aircraft has huge wings. the engines move the wings forward at a high enough rate of speed for the wings themselves to create enough lift to get the aircraft into the air. if the aircraft slows down too much, the wings cannot create enough lift and the aircraft stalls and drops until moving again at a high enough rate of speed to create enough lift to get flying again.
as i understand, some of you think an aircraft flies by the thrust of the engines. you would need thrust equal to the weight of the aircraft to get off the ground. can you imagine the amount of power it would take to lift a 747 straight up off the ground? holy shit! you also wouldn't need wings, you could manuver the aircraft with thrust vectoring or varying the angle of attack of the blades like a helicopter.
a few examples:
the harrier jet. the engines produce enough THRUST to lift the aircraft straight up off the ground.
the osprey: the engines spin the blades, which are basically wings, at a high enough rate of speed to create enough LIFT to get the osprey off the ground. In forward flight, the blades are still angled slightly up to retain altitude. the wings are there to keep the blades from wacking into each other, and to facilitate manuverability. A helicopter works in the same way. except without the wings.
amirite?
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